The present invention relates to modified copolymers of ethylene and of at least one .alpha.-olefin and to a process for their preparation.
It is possible to polymerize ethylene under a very high pressure and at elevated temperature, in the presence of a free-radical initiator. Such a reaction results in a polymer known as radical low-density polyethylene. Irrespective of its melt index, this polymer generally exhibits a tolerable stretchability in the manufacture of tubular casings, and a mediocre tear strength. Good stability of the bubble produced in extrusion-blowing is obtained and the polymer possesses an excellent processability. Good processability is defined as the absence of premature fracture of the extrudate ("melt fracture") combined with low power consumption during the extrusion.
Substantial improvements in the stretchability and tear strength have been made by preparing copolymers of ethylene and at least one .alpha.-olefin, for example, propylene or but-1-ene, in the presence of a Ziegler-type catalyst system. During extrusion-blowing of such copolymers, however, the bubble may become unstable, and the holes have a spontaneous tendency to grow larger in the event of perforation of the bubble. Premature fracture of the extrudate during extrusion is also found.
French Pat. No. 2,132,780 describes copolymers of ethylene with an .alpha.-olefin. These copolymers are modified in an extruder by a free radical initiator that is present in quantities between 0.005 and 5 weight percent, in relation to the polymer to be modified, at a temperature of 204.degree. to 345.degree. C. and under a pressure of 14 to 350 bars. Polymers modified in this manner, when compared to unmodified polymers, all exhibit a reduced molecular weight, a fluidity index augmented by at least 50 percent, and a diminished molecular weight average.
British Pat. No. 1,043,082 describes an intimate mixture of ethylene/.alpha.-olefin copolymers with 0.001 to 10 weight percent of a free-radical initator, at a temperature of 60.degree. to 300.degree. C., preferably in an extruder.
German Pat. Nos. 1,301,540 and 1,495,285 describe the treatment, between 50.degree. and 250.degree. C., of a solution or suspension of a polyolefin in an inert solvent by a free-radical initiator comprising 0.005 to 20 weight percent of the polyolefin. The treatment is carried out in the presence of a sensitizer such as, for example, sulfite, thiosulfate, hydrazine, or mercaptan. This procedure lowers the reduced viscosity of the polyolefin from a range of 1-20 to 0.3-5.
British Pat. No. 1,332,859 describes a continuous process for the polymerization of ethylene in at least two zones of a polymerization reactor. The process comprises continuous passage of the ethylene through a first polymerization zone under a pressure of not less than 1,600 bars and at a temperature not lower than 125.degree. C., in the presence of a free-radical initiator. Then, the reaction medium containing the polymer and the unreacted ethylene, with or without additional ethylene, passes through a second polymerization zone under a pressure of at least 1,600 bars and at a higher temperature in the presence of a Ziegler catalyst. The combined temperature and pressure conditions in each zone are such that the polymerization takes place in a single liquid phase and the reaction medium remains in the form of a single liquid phase until it is removed from the last zone of the reactor.
French Pat. No. 2,202,898 describes a process for polymerization of ethylene under a pressure of more than 1,000 bars and at a temperature between 220.degree. C. and 270.degree. C. by means of a catalyst system comprising titanium trichloride and an alkyl-aluminium, and at least one compound chosen from among titanium alcoholates and silicone oils. The residence time of the catalyst system in the polymerization reactor is less than 150 seconds.